The present invention relates generally to gas inflators for automotive passenger restraint systems, and more specifically, to a gas filter for improved filtration of solids and cooling of combustion gases.
The composition of the gas stream emanating from an automobile airbag inflator is subject to strict requirements to avoid toxicity concerns. Generally, solid propellant gas generators produce unacceptable byproducts which must be removed from the gas stream prior to exiting the gas generator. Due to the high temperatures involved in burning solid propellants, many of the unacceptable byproducts are in the form of liquids or gases which are difficult to remove unless otherwise cooled to the point where the undesirable byproducts convert to solids, which can be filtered out, or to liquids, which solidify in contact with cool surfaces of the gas generator.
The conventional approach to solving the aforesaid problem has been to direct the hot propellant gases directly into a coolant/filter mass and rapidly cool the gases down in a single step to the point where the undesirable solid liquid byproducts are removed. However, a problem is presented by this approach in that rapid cooling of the gases may stabilize the gas combustion equilibrium in a manner that leads to unacceptably high levels of undesired gases.
For example, in airbag inflators, low levels of NO and CO in the effluent gases are mandated. When a stoichiometric propellant containing N, C and O is burned, the quantity of NO and CO produced is a function of the propellant combustion temperature. More CO and NO is formed at higher temperatures. If, as in a conventional system, the combustion gases are quenched in a single step to a temperature at which the gas reaction rates are reduced to essentially zero, combustion will not go to completion and unacceptably high CO and NO levels, resulting from the combustion temperature equilibrium condition, may be produced.
Multistage filters that cool the gases in stages have been designed to address the problems described. However, due to high temperatures, dimensional instability caused by burning and thermal shock is problematic with regard to current filtration designs.
For example, certain multistage cylindrical filters incorporate an expanded metal mesh as an outer filtration layer. The mesh is held in place by a longitudinally welded seam that creates a greater diameter across the filter when compared to other areas on the circumference. When the filter is inserted into a supporting housing or body, a critical radial annulus is required between the outer wall of the filter and the inner wall of the body, thereby ensuring uninterrupted egress of the gases through ports in the body. The connecting seam of the expanded metal mesh reduces the annular region between the seam and the body. Upon gas generant combustion, the heat causes the metal mesh to expand into the radial annulus, thereby further reducing the annular region between the seam and the housing. It has been found that when the area of the seam expands, it will often inhibit radial and circumferential gas flow at the expansion point and eliminate homogeneous gas flow through the filter. As known in the industry, this may result in "filter burn" and rupture of the filter at the point where the gases are unevenly forced out. In certain designs, this may further result in uneven inflation of the airbag and/or explosive potential due to pressure buildup.
Other multistage filters incorporate ceramic heat sinks. Ceramic is an excellent heat sink, however, ceramic must be handled with special equipment during manufacture and assembly. In addition, when heated upon propellant combustion, certain ceramic compositions have been found to liberate undesirable gases.
The weight of many of the materials used in certain multistage filters is yet another disadvantage. Heavyweight materials add to the weight of the airbag inflator as a whole, and also complicate handling during the manufacturing process.